1. Field of Invention
The present invention relates generally to a method and apparatus for removing sludge deposits from the secondary side of nuclear steam generator systems. More particularly, the present invention provides a remotely-operated high-pressure water-jet sludge removal system for pressurized-water reactor steam generators.
2. Description of Related Art
In nuclear power plants, nuclear steam generators serve as large heat-exchangers for generating steam which is used for driving turbines. A typical nuclear steam generator has a vertically oriented outer shell containing a plurality of inverted U-shaped heat-exchanger tubes disposed therein to collectively form a tube bundle. The U-shaped tubes are arranged in a triangular-pitch or square-pitch tube array to form interstitial gaps and intertube lanes that are approximately 1/10 to 4/10 inch wide. In some designs, a centrally located, untubed region extending longitudinally along the central vertical axis of the heat exchanger is defined by the elongated portions of the innermost U-shaped tubes.
A plurality of horizontally oriented upper annular support plates, or in some designs "eggcrate" supports, are provided at periodic intervals for arranging and supporting the U-shaped tubes. Each support plate or eggcrate support contains a triangular- or square-pitch array of holes or openings therein for accommodating the elongated portions of the U-shaped tubes. The upper support plates or eggcrate supports are positioned in relation to one another so that the holes thereof are aligned, thereby allowing the elongated portions of the U-shaped tubes to be accommodated within the holes. The height of the U-shaped tubes may exceed thirty-two feet. A steam generator typically includes six to eight or more support plates and/or eggcrate supports, each horizontally disposed and vertically separated at three- to five-foot intervals. Additionally, tie rods and wrappers may be used for providing further support to the U-shaped tubes by supporting the annular support plates and/or eggcrate supports.
A tubesheet spaced below the lowermost eggcrate support separates a lower primary side from an upper secondary side of the steam generator. A dividing plate cooperates with the lower face of the tubesheet to divide the primary side into an entrance plenum for accepting hot primary coolant from a nuclear core and an exit plenum for recycling lower temperature primary coolant to the reactor for reheating. The entrance and exit plenums are connected by the U-shaped tubes. Primary fluid that is heated by circulation through the core of the nuclear reactor enters the steam generator through the entrance plenum. The primary fluid is fed into the U-shaped tubes, which carry the primary fluid through the secondary side of the steam generator. A secondary fluid, generally water, is concurrently introduced into the secondary side of the steam generator and circulated through the interstitial gaps between the U-shaped tubes. Although isolated from the primary fluid in the U-shaped tubes, the secondary fluid comes into fluid communication with the peripheral surfaces of the U-shaped tubes. Heat is consequently transferred from the primary fluid to the secondary fluid, which, in turn, converts the secondary fluid into steam that is removed from the top of the steam generator in a continuous steam generation cycle. The steam is subsequently circulated through standard electrical generating equipment. The cooled primary fluid exits the steam generator through the exit plenum, where it is returned to the nuclear reactor for reheating.
The secondary fluid entering the steam generator often includes undesirable impurities or chemicals. The principal impurities are iron, copper, and hardness species such as calcium and magnesium. Due to the constant high temperature and severe operating environment, these impurities are left behind in the steam generator and manifest themselves in the form of a corrosive sludge mainly comprised of iron oxides, copper oxides, copper metal, and insoluble hardness species. The sludge accumulates on the outer peripheral surfaces of the U-shaped tubes, the support plates, the lower eggcrate supports, and within the interstitial gaps formed by the tubes and the tube supports. It is not uncommon for thousands of pounds of sludge to accumulate after only several years of plant operation. Tube surface deposits account for approximately 80 to 85 percent of the sludge in a typical steam generator. As the sludge accumulation on the tube bundle and the tube supports increases, the heat transfer efficiency of the steam generator correspondingly decreases. Moreover, corrosion of the heat exchanger U-shaped tubes and potential stress corrosion cracking in the U-shaped tubes raises concerns over leakage of radioactive primary fluid and resulting contamination of the secondary fluid.
Thus, periodic removal of the sludge from the steam generator is an important step towards maximizing the heat transfer efficiency of the steam generator and alleviating concerns over corrosion. To remove the sludge, several cleaning apparatuses and methods have been proposed. Examples of prior art cleaning methods include chemical cleaning, pressure pulse cleaning, and sludge lancing. Chemical and pressure pulse cleaning are considered unfavorable because their costs tend to be excessive. In addition, chemical cleaning can advance the corrosion of the steam generator structure and pressure pulse cleaning is only marginally effective for removing deposits.
Conventional sludge lancing involves directing a high-powered jet (about 1500 to 15,000 pounds per square inch (psi)) of pressurized water at sludge located on the tubesheet, where approximately 15 percent of the sludge is located. In many nuclear steam generators in service today, there are two- to six-inch diameter hand holes located in the outer shell about the periphery of the tubesheet that provide access to the secondary side of the steam generators at the tubesheet elevation. The hand holes provide access to an untubed corridor, also known as a blowdown lane, that extends along a diameter of the steam generator and in some designs passes through a central untubed portion thereof. The untubed corridor is approximately four inches wide. Vertically oriented tie rods often bisect the untubed corridor, dividing it in two unencumbered free lanes having respective widths less than approximately 13/4 inches. Conventional sludge lancing systems are introduced into the steam generator through the hand holes, thereby allowing for water jet nozzles to be positioned along the blowdown lane. The sludge lancing jets are moved along the blowdown lane and aligned with the gaps formed by the tube array. Pressurized water discharged from the lancing jets impinges upon the sludge deposits to loosen them. Once dislodged by the water jets, the tubesheet sludge deposits are collected by a suction system. In other conventional tubesheet sludge lancing designs, a mobile system is deployed at the periphery of the tube bundle, and water jets are directed from the tube bundle periphery radially inward toward the center of the bundle. The disadvantage of these conventional sludge lancing systems is that they can only target sludge deposits at the lowest region of the steam generator. The majority of the deposits located on the secondary side are not accessible, since hand holes typically are not present in the upper portion of the outer shell.
Attempts to provide a system that may be introduced through a lower hand hole for cleaning the upper portion of the secondary side have heretofore proven to be ineffective or infeasible. For example, U.S. Pat. No. 5,265,129 issued to Brooks et al. discloses a support plate inspection device (SID) that includes a horizontal boom that extends along a blowdown lane, a vertical telescoping member attached to the distal end of the horizontal boom that extends upwardly into the secondary side of a steam generator, and a cleaning nozzle and video camera attached to the upper distal end of the vertical member. However, those skilled in the design of remotely operated robotic systems would conclude that the SID system may not be able to display a high load-bearing capability and could become inherently unstable as a result of a variety of torques, loads, and moments placed on the telescoping member. That is, when the vertical member is arranged in its extended position, it is susceptible to flexure, buckling, and bending caused by any eccentricity of the load thereon or by reaction forces imparted on the system by the water jet discharged from the attached cleaning nozzle. The inadequacy of a telescoping design is related to the need to use thin-walled tubing to form a suitably small set of nested cylinders (e.g., the collapsed system must exhibit an overall diameter of less than 2 to 3 inches). As a result, the SID system can damage the tube bundle or even become permanently lodged within the steam generator. In addition, the SID system requires a sufficient amount of clearance between the tubesheet and the next highest support member for rotational up-ending so as to place it in a configuration suitable for vertical extension. Only some steam generator designs exhibit sufficient spatial dimension to permit this movement.